1. Field of the Invention:
The present invention relates to a multistage vacuum pump including a plural set of a plural lobe type vacuum pumps arranged on a common shaft for rotors and in a common casing.
2. Prior Art:
In general, in order to obtain a high vacuum, a plurality of single-stage vacuum pumps are arrayed in series. Particularly of late, however, a multistage vacuum pump having a plurality of rotors provided on a common shaft has been employed so as to provide a miniaturized vacuum pump. Its application is diversified including a case where a sublimable gas is handled. The sublimable gas has a property to change, as shown in FIG. 6, from a gas to a solid or from a solid to a gas according to a change in the state of pressure and temperature.
In case a sublimable gas is handled by a multistage vacuum pump as referred to above, however, there, entailed a problem that a solid material is produced within the pump as the gas is compressed successively from an inlet port and arrives at a discharge port, and this greatly decreases the lifetime of the pump. Particularly when the solid material adheres to fine gaps in a rotor portion (compression part), the pump stops instantaneously. Thus, a pump body must be disassembled, cleaned and reassembled in each such occasion, which involves much time and troublesome labor.